In an elastic collision of two particles,which of the following quantities is conserved?

$A$ ball of mass $1 \ kg$ moving along the $X$-direction collides elastically with a stationary ball of mass $m$. The first ball (mass $= 1 \ kg$) recoils at a right angle to its original direction of motion. If the second ball starts moving at an angle $30^{\circ}$ with the $X$-axis,the value of $m$ must be (in $kg$)

Which of the following statements is true?

Six steel balls of identical size are lined up along a straight frictionless groove. Two similar balls moving with a speed $V$ along the groove collide with this row on the extreme left end. Then:

$A$ ball of mass $m$ suspended from a rigid support by an inextensible massless string is released from a height $h$ above its lowest point. At its lowest point, it collides elastically with a block of mass $2m$ at rest on a frictionless surface. Neglect the dimensions of the ball and the block. After the collision, the ball rises to a maximum height of

When two particles collide elastically,then impulsive force can't change